In your .vimrc file do
set grepprg=zsh\ -c\ 'grep\ -nH\ $*'
zsh allows recursive glob expansions so you can do do from your vim command line
grep "def foo" **/*.rb
of which the results will end up in you quickfix buffer. This is several orders of magnitude faster than the standard vimgrep command.
To see the quick fix buffer you need to do
at the vim command line